Packet switching networks transmit digital information between sources and destinations known as end points. The end points can be directly connected to the packet network or they may be indirectly linked through an intermediary non-packet network such as a local loop of a telephone network. When the end points are directly connected to the network, a source end point creates packets, each packet having a data portion and a routing information portion. The packets are then transmitted to an input port of the packet network. Similarly, end points that are not directly connected to a packet network transmit non-packetized data which is subsequently packetized at an input port of the packet network.
The packet network includes a plurality of interconnected nodes which receive the packets in succession; and based on the routing information within each packet, each node selects a path to another node or a network output port. As packets traverse a packet network from input ports through intermediate nodes to output ports, they may become unusable or lost. In general, congestion at network nodes is a main cause of packet loss. As buffer space is quickly used up at the nodes, an overload condition may occur, resulting in either a delay in transmission or loss of data.
From the standpoint of the destination end point, packet loss causes inaccuracies in the data signal stream received from the source end point. In some cases, when packets are dropped, the received data stream reveals a gap in the received signal, which is easy to detect. In other cases, packets may be immediately substituted for the unusable packets, concealing the packet loss. The substitute packets may be the last usable packet or a new packet may be generated according to an algorithm that uses information from the last packet and possibly the next packet to estimate the lost data by interpolation.
To improve the quality of data transmission in packet networks, techniques have been developed to identify packet loss. One technique uses packet monitoring systems connected to nodes of the packet network to transmit special packets to measure packet loss. In another technique, packet loss is detected by monitoring normal packets in the network and checking the sequence numbers of the packets to determine if any packets have been lost. These techniques require access to the packet network being evaluated and suffer from the further disadvantage that they may not identify packets lost after the point at which they are connected to the network. As a result, they may not provide a fully accurate measurement of end-to-end network performance.